Synergistic Effects of Cation and Anion in an Ionic Imidazolium Tetrafluoroborate Additive for Improving the Efficiency and Stability of Half‐Mixed Pb‐Sn Perovskite Solar Cells

Abstract

Narrow‐bandgap mixed Pb‐Sn perovskite solar cells (PSCs) have great feasibility for constructing efficient all‐perovskite tandem solar cells, in combination with wide‐bandgap lead halide PSCs. However, the power conversion efficiency of mixed Pb‐Sn PSCs still lags behind lead‐based counterparts. Here, additive engineering using ionic imidazolium tetrafluoroborate (IMBF4) is proposed, where the imidazolium (IM) cation and tetrafluoroborate (BF4) anion efficiently passivate defects at grain boundaries and improve crystallinity, simultaneously relaxing lattice strain, respectively. Defect passivation is achieved by the chemical interaction between the IM cation and the positively charged under‐coordinated Pb2+ or Sn2+ ions, and lattice strain relaxation is realized by lattice expansion with the intercalation of BF4 anions into the perovskite lattice. As a result, the synergistic effects of the cation and anion in the IMBF4 additive greatly enhance the optoelectronic performance of half‐mixed Pb‐Sn perovskites, leading to much longer carrier lifetimes. The best‐performing half‐mixed Pb‐Sn PSC shows an efficiency above 19% with negligible hysteresis, while retaining over 90% of its initial efficiency after 1000 h in a nitrogen‐filled glovebox and showing a lifetime to 80% degradation of 53.5 h under continuous illumination.

High‐Performance Tin–Lead Mixed‐Perovskite Solar Cells with Vertical Compositional Gradient

“…The unencapsulated devices were stored under continuous white-light illumination (100 mW cm −2 ) in a N 2 glovebox and measured under ambient condition at specific time interval. The target devices can retain about 90% of their initial efficiency after 150 h light soaking, which are among the most stable Sn-Pb mixed PSCs reported up to now, [62][63][64] while the control without HBA remains only about 60% of their initial values under the same condition. The enhanced lightsoaking stability can be ascribed to the presence of HBA-SnF 2 complex at grain boundaries to passivate defects.…”

Optoelectronic Properties of Low‐Bandgap Halide Perovskites for Solar Cell Applications

“…[91] Very recently, Kim et al used ionic imidazolium (IM) tetrafluoroborate (IMBF 4 ) as a multifunctional additive with FA 0.5 MA 0.5 Pb 0.5 Sn 0.5 I 3 perovskites, where the IM cation is proposed to chemically interact with the positively charged undercoordinated Pb 2+ /Sn 2+ ions and the intercalation of BF 4 anions results in relaxing the lattice strain. [92] As a result of these synergistic effects, all the photovoltaic parameters were improved, resulting in a champion efficiency of 19.1% for single-junction low-bandgap PSCs. In a similar vein, zwitterionic formamidine sulfinic acid (FSA) has also been found to efficiently passivate defects (possibly both cationic and anionic defects) at the grain surfaces in mixed Pb-Sn perovskite films, enabling an efficiency of 25.6% for small area (0.049 cm 2 ) all-perovskite tandem solar cells.…”

Inch-Sized Thin Metal Halide Perovskite Single-Crystal Wafers for Sensitive X-Ray Detection

“…The hole-only devices with the structure of ITO/ PEDOT: PSS/single crystals/Au were fabricated (Figures 4C,D inset). The carrier mobility and trap density are calculated based on Equation 1 and Equation 2, respectively (Kim et al, 2020):…”

“…The hole-only devices with the structure of ITO/PEDOT: PSS/single crystals/Au were fabricated ( Figures 4C,D inset ) . The carrier mobility and trap density are calculated based on Equation 1 and Equation 2 , respectively ( Kim et al, 2020 ): where J D is the dark current density, ε is the dielectric constant of perovskite single crystals, ε 0 is the dielectric constant of vacuum, μ is the mobility, V b is the bias, and L is the crystal thickness ( Zhang et al, 2018 ). where V TFL is the voltage at which all the traps are filled, e is the elementary charge, and n t is the hole trap density.…”